Ink jet printing is a non-impact method that in response to a digital signal produces droplets of ink that are deposited on a substrate such as paper or transparent film. Ink jet printers have found broad application as output for personal computers in the office and the home. There are several classes of ink jet printer, for instance thermal drop-on-demand printers, piezo drop-on-demand printers, and continuous ink jet printers.
Continuous ink jet recording has several advantages in that it produces low noise and can obtain a high-resolution recorded image at a high speed by utilising a highly integrated head. It is common to use a solution obtained by dissolving one or more water-soluble dyes in water or a solvent mixture of water and an organic solvent as the ink for such an ink jet recording method. In addition to the properties required of the ink for good printing performance, there are other requirements affecting the start up of the printing apparatus after it has been turned off for any length of time. For example, firstly, any ink which dries on the orifice plate or other surfaces should redissolve rapidly when the apparatus is restarted, and secondly, it is also important that the dried ink film should have adequate electrical resistivity to prevent shorting out electrodes in the system when an attempt is made to restart the printhead. Current state-of-the-art aqueous ink jet inks which utilize water soluble dyes achieve acceptable performance in these respects.
Several suitable water soluble dyes which have been perfected for ink jet use are known, but there are some deficiencies to the use of these dyes. For instance they remain soluble in water on the printed page and are therefore subject to print defects such as feathering and to blurring and leaching by water, for example rain water and coffee. Furthermore, many of the dyes in use have poor light fastness and fade even on exposure to fluorescent lighting used in offices.
Consequently there is interest in the use of a black pigment such as carbon black in inks for continuous ink jet printers. The term pigment describes a colorant which is substantially insoluble in the aqueous ink medium. It is expected that prints produced using a pigmented ink will display better light stability and also improved fastness to washing and weathering as the pigment is dispersed rather than dissolved.
However the use of pigments rather than dyes creates some complications in the use of the ink. For instance, redispersion rather than redissolvability must be achieved to have good system start-up. In addition, pigmented black inks based on carbon black dry down to provide films which are electrically conductive, presumably due to the interparticle contact at the bare carbon particle surface. This means that when a continuous ink jet printhead is shut down, the films formed from the dried inks on surfaces in the apparatus tend to short out electrodes in the system and that start up performance is poor.
Thus there is a need to increase the electrical resistivity of dried ink films from pigmented black inks. To a certain extent the problem may be overcome by reducing the concentration of the black pigment, but the density of the printed image then becomes inferior to that of the image printed by using a dyed ink and also the image tends to become brown shaded or warm toned. Although under certain circumstances toned images may be desired, a neutral or nearly neutral shade is normally preferred. Further, it is known according to U.S. Pat. No. 5,609,671 that the use of inks incorporating dispersions prepared from surface treated or chemically modified black pigments will increase the electrical resistivity of dried ink films. However this is normally insufficient to overcome the problem. Furthermore, it is also known that the addition of various polymers to pigmented inks will increase the dried ink film resistance. For instance U.S. Pat. No. 6,203,605 discloses an ink jet ink composition comprising a liquid vehicle, a chemically modified pigment dispersion, and also incorporating a polymer to provide a synergistic effect in increasing the dried ink film resistance values. However the amount of polymer which may be added to the ink is limited as the viscosity is otherwise excessive for satisfactory use.
It is seen then that there is a need for an improved pigment based black ink suitable for use in continuous ink jet printers. We have developed an ink formulation which provides acceptably resistive dried ink films for use in continuous ink jet printers.
European Patent Application 0 899 311 A discloses use of a combination of carbon black with an organic pigment in an ink jet ink. U.S. Pat. No. 5,803,958 discloses use of carbon black in combination with cyan and magenta pigments. However neither of the patents disclose inks that are used in continuous ink jet printers nor do they disclose the use of surface treated carbon black with an organic pigment.